The technique of shearing interferometry, a process which is commonly referred to as shearography, typically involves the interference of two laterally displaced images of the same object to form an interference image. Conventional shearographic methods include taking a first interference image (or baseline image) be taken while the object is in an unstressed or first stressed condition and another interference image while the object is in a second stressed condition. Comparison of these two interference images, for example, by methods of image subtraction, reveals information about the presence of certain defects in the composite tire or casing material in a single image called a shearogram. In particular, shearography has been shown to be useful to detect defects in vehicle tires, especially retread vehicle tires.
In conventional electronic shearography machines, a tire or tire casing is loaded onto an inspection table. The table may include a radiation emitter and receiver assembly that can be raised relative to the table within a central opening of the casing and can be rotated relative to the tire to permit inspection of the entire surface of the tire. Typically, shearography machines include a chamber that can enclose the tire such that a vacuum may be applied to the tire. Most tire defects, such as material layer separation, will result in air being trapped within the tire material. This trapped air will cause bulges to appear when the tire is subjected to a vacuum or dimples when the tire is subjected to positive gage pressure within the enclosure. These bulges or dimples can then be detected by the shearographic process. One example of a known shearographic machine can be found in U.S. Pat. No. 6,791,695 (the '695 patent), which is incorporated herein in its entirety by reference.
In typical shearographic inspection machines, such as the machines described in the '695 patent, a clamshell enclosure is used to house the tire during the inspection process. With the clamshell open, loading and unloading a tire into and out of the machine involves placing the tire onto a moveable tray that tilts into and out from the clamshell enclosure. The moveable tray includes pins that center or otherwise position the tire within the enclosure to provide an appropriate distance between the various surfaces of the tire and the shearographic emitter/receiver. When the tray is tilted out of the enclosure, an operator may manually load and unload the tire from the tray.
In the known inspection machines, the handling time for each tire from loading into the machine to unloading out of the machine can take between forty seconds to several minutes. During this time, the inspection machine is not operating to inspect tires, which can result in relatively high “dead” times and relatively low inspection rate throughputs for the machines.